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The first element in group one, Lithium, and the first in group two, Beryllium, behave differently to other members of their groups. Their behaviour is like the second element of the next group. So lithium is similar to magnesium, and beryllium is similar to aluminum.[source?]

In the periodic table this is known as a Diagonal relationship. The diagonal relationship is because of similarities in ionic sizes and charge/radius ratio of the element. The similarity between lithium and magnesium is because of their similar sizes:[source?]